If T cell clones are so diverse, what prevents anti-tumor immune response?

Identifying cancer and pathogen-specific epitopes or TCRs may sound intuitive, but it is a futile approach. Diversity of T cell or B cell clones guarantees that the adaptive immune system will always have relevant clones to detect cancer or pathogen.  

Epitopes have no meaning attached to them with one exception.  It is when the said epitope is self. Each body will have different sets of self-epitopes. Every self-epitopes relevant for host's survival are encoded in the thymus, and thymic Tregs are trained to prevent any T cell activity against those epitopes in the periphery. This is called tolerance, and it is antigen[epitope]-specific. 

Then what prevents effective responses to cancers or pathogens? It is commonly but mistakenly believed that Tregs prevent effective T or B cell responses to cancers or pathogens. But Tregs only prevent anti-self response, and it is epitope-specific action.  So, by definition, if Tregs do their job as required, we cannot blame them. But it has nothing to do with cancer or pathogens, which obviously have other epitopes different from self, we call nonself. So, if cancer cells or pathogens express nonself epitopes that are always detected by adaptive immune system, why not everyone can fight it off effectively?

This is because T cells themselves prevent it. Yes, T cells, not Tregs, prevent effective response to cancers or pathogens in certain conditions. What are those conditions? These are condition when polarized T helper cells prevent other T cells functions. Polarization is a pathological state. A Polarized T cell's effect on other T cells is epitope non-specific, meaning, a polarized T helper cell specific to cancer or pathogen nonself epitope A will prevent T cells specific to epitope B, C, D, E, F, etc., to function properly. It is exactly Treg's job to shut down those polarized T helper cells to allow other T cells to manifest their functions and get rid of either cancer or pathogen. And Treg do it, as we already said, epitope-specific manner.

For this reason, it is not so important to identify any cancer or pathogen-specific nonself epitope, but rather to identify an epitope that could activate polarized T helper cells.

To make things even more complicated, one may ask if Tregs are self-specific and act epitope-specific manner, how can Tregs shut down polarized T helper cells that are nonself-specific? It is possible because Tregs are cross-reactive and can inhibit only those polarized T helper cells which share TCR specificity with Tregs.

In other words, control of self-tolerance and control of effective anti-nonself response are one and the same.  

These are 3 papers that together provide a full discussion related to the SPIRAL model we have developed to explain how Tregs work within an adaptive immune system:

 

 

Concurrent cross-reactivity of microbiota-derived epitopes to both self and pathogens may underlie the "Hygiene hypothesis"  

 

https://onlinelibrary.wiley.com/doi/10.1111/sji.12708 

 

Could cross-reactivity rescue Foxp3+ regulatory T cell precursors from thymic deletion? 

 

https://onlinelibrary.wiley.com/doi/10.1111/sji.12940

 

 

 

Microbiota-Specific Foxp3+ Regulatory T Cells Could Control Pathological T Helper Responses

 

https://www.dl.begellhouse.com/journals/2ff21abf44b19838,40a32b197f7156bc,7988864072d2c7c6.html

 

 

   

A specific bacteria-infecting virus, bacteriophage, found in gut microflora, augments anti-tumor T cell immunity

Molecular mimicry between microbial and host's antigens could contribute to autoimmunity but also to the protection against tumors through epitope cross-reactivity. A new study in journal Science indicates that those cross-reactive epitopes could come from viruses that infect endogenous microbial species. 

In this study the authors made a surprising observation that only certain Enterococcus hirae microbial strains (E. hirae 13144 or IGR11) augmented anti-cancer effect in experimental cancer model.

 

 

Next, the authors showed that this biological activity was linked to one dominant epitope, TSLARFANI, derived from TMP protein that originated in 39.2-kb prophage only in those specific E. hirae strains. Mice immunized with heat-inactivated E. hirae 13144 strain, or peptide TSLARFANI, or irrelevant E.coli engineered to express TMP, all augmented anti-cancer effect. 

 

 

Mechanistically, the authors showed that epitope, GSLARFRNI, derived from cancer cells used in these experiments, was recognized by the same CD8 T cells which labeled with TSLARFANI epitope tetramers confirming cross-reactivity between these 2 epitopes.

 

 

In summary, this study suggests that microbiota and bacteriophages they carry represent new modality in fight against cancer. In this study overall anti-tumor effect is modest but we need to take into account that this is an effect of just one cross-reactive epitope in one type of MHC inbred mice, and it is likely that many other epitopes will be involved in outbred species such as humans. However, it is still extremely hard to do such analysis in humans in real world scenario due to lack of exact knowledge about human microbiota strains and poor reliability and performances of available bioinformatics approaches. However, once the mechanistic principles underlying anti-cancer effects are uncovered and accepted, then it is much easier to move the field forward.

posted by David Usharauli

Is it possible to engineer Foxp3+ Tregs from primary T cells?

Here is the most recent paper that claims that they can do it. It was published in Science Translational Medicine. This is fairly respectable journal run by Science. This research group is so confident in their data that they even set up a new biotech company to commercialize their approach. I am going to examine how strong are their claims.

Here is a short description what they did: they used a combination of specific nuclease (TALEN) and virus (AAV6) to insert a new promoter into Foxp3 gene in an in vitro activated T cells. They called these Tregs edTregs.

insertion of MND promoter

The edTregs displayed very similar functionality known to occur in thymus-derived Tregs (tTregs) such as no or limited expression of IL-2 and other cytokines in an in vitro stimulation assays. 

The edTregs were suppressive towards effector T cells in a proliferation assay as should be expected from tTregs. Moreover this essential function required endogenous Foxp3 activity as edTregs from IPEX individuals with a defective Foxp3 gene did not show suppression.

proliferation assay

However, edTregs were significantly different from tTregs in Treg-specific demethylated region (TSDR). The point is it is now accepted that Treg identity is not established solely because of Foxp3 expression but requires specific and selective epigenetic modification within and outside of Foxp3 gene. Nonetheless, in in vitro assays, edTregs behaved as bona fide tTregs. 

What about in vivo? There the story gets a little bit murky. The authors used two models to assess edTregs in vivo. First, they co-transferred edTregs with effector T cells into immunodeficient mice to assess if edTregs could prevent graft versus host disease (GvHD). They do see reduction of mice mortality with edTregs.

However, there are some inconsistency between experiments describing GvHD model. In one set of experiments it produced 100% lethality by day 21 (see below, red line) while in other set of experiments it produced only 20% lethality (see above, red line). 

Such inconsistency casts doubts about edTregs ability to inhibit effector T cells in vivo and could explain why the authors did not see much difference in GvHD scores with or without edTregs (see below).

                                                                                                                      

This could also explain why the authors did not see improvement in brain inflammation in mice EAE model when co-transferring antigen-specific edTregs with effector T cells.

In summary, this paper has done a lot of interesting in vitro work trying to convince us that their edTregs work as intended. However, in vivo work lacks consistency. It is not surprising. It has been known for some time now that Tregs behave differently in vitro vs. in vivo. Suppression in vivo appears to be strictly antigen-specific phenomenon unlike in vitro where it could be observed antigen-nonspecific manner (even though Treg activation in itself still require presence of cognate antigen). 

posted by David Usharauli